The present invention relates to a method for location of a fault which has occurred on one of the lines or feeders in a distribution network where the location is performed with the aid of measured values of the common supply voltage of the lines and the currents of the lines after the occurrence of a fault; calculating the equivalent positive-sequence impedance Z1kf and zero-sequence impedance Z0kf of the network in a pre-fault steady state for all M nodes based on knowledge of the configuration and topology of the network, and obtaining, via a superordinate protection system, information about which line has become faulty and which type of fault has occurred. The invention also relates to a device for carrying out the method.
Fault location in distribution networks (DN), cable or overhead, is normally an integral part of superordinate protection systems relating to faults on circuit breakers, contactors, relays etc. With the aid of various protection, monitoring and so-called expert systems, the faulty line may be determined. In the document, the word line is used, but in this context it is to be understood that it equally applicable to feeders or cables, and combinations thereof.
The state of the art as regards fault location in a DN comprises two fundamentally different methods. One of the methods is based on the provision of a fault locator on each line, which entails heavy investment costs, and the other method comprises measuring centrally the voltage and the sum current for all the DN lines in the DN station.
The latter method involves a plurality of problems, which make it difficult to obtain a relatively reliable measure of the distance to the fault:
in connection with fault location, assumptions are often made that the current in a faulty line is equal to the difference between measured current after and prior to the occurrence of a fault, which introduces a certain error in the determination of the distance;
if the line comprises motor drives, this may lead to power being fed into the DN, and such feeding of power is difficult to compensate for;
the line may comprise one or more substations and closed loops;
a fault locator is programmed for a given number of branches with respective loads at given distances from the DN. Since connection and disconnection of parts of the line may occur at different times, it is important to update programmed data of the network configuration and topology.
In an article entitled xe2x80x9cDetermining Locations on Faults in Distribution Systemsxe2x80x9d, Developments in Power System Protection, Mar. 25-27, 1997, Conference Publication No. 434, IEE 1997, a method for determining distance is described, wherein a central measurement of the voltage and the sum current for all the lines is performed. The fault-located line may have a plurality of distributed branch points, nodes, where also some branches have parallel loads. The starting-point is voltage and current measured at the DN station prior to and after the occurrence of a fault, whereupon the respective positive-sequence components are determined. It is assumed that the data of the line between each node and the load at each node prior to a fault are known.
A first assumed value of the distance to the fault is determined on the basis of the positive-sequence impedance of the remote end prior to the fault. The positive-sequence components of current and voltage at the fault node after the occurrence of the fault are then used for determining the first calculated value of the distance to the fault. These two values are compared with each other, and if the difference is greater than a least value set in advance, a new assumption is made as to between which nodes the fault is located, based on the value now calculated. This provides a new load model and a second calculated value of the distance to the fault. This value is then compared with the first calculated value, which comparison may result in an additional number of iterations until the difference value between two consecutively calculated values lies within the permissible values. The method does not permit fault location in case of a three-phase fault.
One way of making the determination of the distance to a fault when performing measurement on the relevant faulty line is clear from an article entitled xe2x80x9cAn Interactive Approach to Fault Location on Overhead Distribution Lines with Load Tapsxe2x80x9d, Development in Power System Protection, Mar. 25-27, 1997, Conference Publication No. 434, IEE, 1997, in which the term xe2x80x9coverhead distribution linesxe2x80x9d relates to an overhead line intended for medium voltages. This article presents a technique and an algorithm for fault location on overhead lines based on determining the difference in voltage prior to and after the occurrence of a fault at an assumed fault point on the line based on voltages measured in the supply station of the line, prior to and after the occurrence of a fault. This voltage is then used for checking the currents in the non-faulty phase at the assumed fault point. Only when the assumed fault point is correct, will the current in the non-faulty phases assume a value near zero. This method does not permit any fault location of a three-phase fault and the voltage measurement must be performed in the supply station of the line in question.
Further problems with fault location in DN""s, are that, in contrary to transmission lines, the distribution networks are usually non-homogeneous, with branches and loads along the line which makes the fault location (FL) accuracy difficult. A general scheme of such a network is presented in FIG. 1. The fault-loop impedance estimated by FL at the substation and used as a direct measure of a distance to fault is corrupted by intermediate loads and branches that makes accurate fault location difficult. Three fundamental factors contribute to this:
a fault-loop as seen from the substation may contain different cable sections with different equivalent parameters what can not be regarded as homogenous circuit, therefore no classical FL methods may be used;
in the case of a DN line, there are often loads located between the fault point and the busbar; since the loads change and are unknown to the FL it is difficult to compensate for them;
resistance at the fault point introduces equivalent fault impedance which value and character depends on the equivalent network parameters beyond the fault, this is also difficult to compensate for.
By means of a method and a device according to the invention, determination of the distance to the fault on a faulty line of a Distribution Network (DN) may be performed, wherein the method takes into consideration the influences of non-homogenities, branches and loads of the DN. Further the method according to the invention is not dependent of where in the network measurements are being made, i e does not depend on if the currents and voltages of each line or branch are measured separately or if the voltage and sum current for all the lines are measured centrally.
The principle of distance determination according to the invention is particularly useful for cable networks but may also advantageously be used for overhead line networks.
The method proposed for this invention overcomes the difficulties discussed above by delivering a method for fault location in distribution networks characterised by the features of claim 1. First, the equivalent positive- (Z1kf ) and zero-sequence (Z0kf ) impedance of the network is computed in pre-fault steady-state for all M nodes of the network based on existing topology, loads and feeder parameters. Second, after the fault, the specific fault-loop parameters are calculated depending on the fault-loop type (phase-phase or phase-ground) and the place of measurements (at the supplying transformer or at the faulty feeder).
The fault location is determined as a result of checking the following set of conditions:
Im(Zek)xe2x89xa60, k=1,2, . . . M (A)
where:                                           Z            _                    ek                =                  {                                                                                                                                                                                              Z                            _                                                                                1                            ⁢                            k                                                    f                                                -                                                                              Z                            _                                                                                1                            ⁢                            f                                                                                                                                                                                        -                                                ⁢                        for                        ⁢                                                  xe2x80x83                                                ⁢                        phase                        ⁢                                                  -                                                ⁢                        to                        ⁢                                                  -                                                ⁢                        phase                        ⁢                                                  xe2x80x83                                                ⁢                        fault                        ⁢                                                  -                                                ⁢                        loop                                                                                                                                                                                                      Z                            _                                                                                1                            ⁢                            k                                                    f                                                +                                                                                                            k                              _                                                        I                                                    ⁢                                                                                    Z                              _                                                                                      0                              ⁢                              k                                                        f                                                                          -                                                                              Z                            _                                                                                1                            ⁢                            N                                                                                                                                                                                        -                                                ⁢                        for                        ⁢                                                  xe2x80x83                                                ⁢                        phase                        ⁢                                                  -                                                ⁢                        to                        ⁢                                                  -                                                ⁢                        ground                        ⁢                                                  xe2x80x83                                                ⁢                        fault                        ⁢                                                  -                                                ⁢                        loop                                                                                            ⁢                                  
                                ⁢                                                      k                    _                                    I                                            =                                                                    I                    _                                    pN                                                                      3                    ⁢                                                                  I                        _                                            p                                                        -                                                            I                      _                                        pN                                                                        ,                          xe2x80x83                        ⁢                                                            Z                  _                                                  1                  ⁢                  N                                            =                                                                    V                    _                                    ph                                                                                            I                      _                                        p                                    -                                                                                    I                        _                                            pN                                        /                    3                                                                                                          (        A        )            
and: Vph - voltage at the faulty phase, Z1f - positive-sequence fault-loop impedance obtained from measurements, Ip, IpN - adequately: fault-loop and residual currents obtained from measurements.
The final distance to fault will be chosen when the condition as in (A) is fulfilled. The method of calculation of the parameters (Z1f,Ip, IpN) depends on the place of measurement (at the substation or the feeder).
With the present invention it is possible to determine the distance to fault in a very accurate and reliable way. Especially in distribution networks including a plurality of different line or cable sections and with branches and loads along the lines, the present invention takes this into account by utilising fault loop approaches depending on the type of fault and going through the consecutive nodes of the network calculating the residual impedance in order to arrive at a distance value.
These and other aspects of, and advantages with the present invention will become apparent from the detailed description and from the accompanying drawings.